1. Field of the Invention
The present invention relates to turbomachinery, and more particularly to a turbine airfoil with structure to secure the airfoil to a rotor disc.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a turbo machine such as an axial flow compressor or an axial flow turbine, it is sometimes necessary to remove and replace a damaged blade or vane. Prior art turbo machines make use of a dovetail slot in the rotor disk in which a similar shaped dovetail root of an airfoil slides within to secure the airfoil to the rotor disk against radial movement. To remove an airfoil from this type of rotor requires significant disassembly of the turbo machine in order to remove the airfoil from the slot. The casing must be removed, and one or more vanes must be removed in order to make space for the blade to be able to slide out from the slot for removal.
One way of solving this problem is disclosed in the U.S. Pat. No. 3,565,547 issued to Hansen on Feb. 23, 1971 which shows a turbo machine rotor construction in which a blade includes a root portion having an externally threaded shank portion, and the rotor disc includes radially extending tapped opening with screw threads such that the blade can be screwed out of the opening and another blade easily installed. In a blade attached to the rotor disk like the Hansen invention, the blade can be easily removed from the rotor disk by unscrewing the blade. One of the problems with the Hansen invention is the use of regular type screw threads typically found on bolts. Most of the shear stress in the threads of the Hansen invention is located at the top two threads. In a turbo machine such as an axial flow compressor, the most efficient performance is established when the AN2 is at a maximum value. With the screw type threads of the Hansen invention, the high rotation speeds required to reach a high performance level would result in stress levels on the threads large enough to cause damage to the threads and result in the blade pulling out from the rotor disk.
Another problem with the Hansen invention is that the blade must be rotated and tightened in the opening to prevent the blade from loosening due to vibrations. The airfoils on the blade must be precisely set such that the angle of attack of the airfoil is positioned properly, especially if a guide vane is used upstream of the blade or if several rows of blades and vanes are used. Due to tolerances in the threads on both the blade root and the opening in the rotor disk, the angle of attack on the airfoil may not be properly aligned when the blade is tightly screwed into the opening.
What is needed in the art of turbo machinery is a threaded connection between the blade (airfoil) and the rotor disk that would allow for the blade to be removed without breaking down the casing, but would also allow for high rotation speeds such that the AN2 is at a maximum value.
It is therefore an object of the present invention to provide for a turbo machine having radially extending airfoils in which the airfoil can be easily removed without disassembling the casing or other turbo machine parts.
It is another object of the present invention to provide for a screw type connection between the airfoil root and an opening in the rotor disk that will allow for high rotation speeds without causing high stress levels in the screw type connection.
It is another object of the present invention to provide for a screw type connection between an airfoil and a rotor disk that includes a locking member to prevent the threaded members from loosening due to vibrations in the rotor disk, and to set the airfoil at the proper angle of attack for maximum performance.
It is another object of the present invention to provide for an opening in the casing of the turbo machine such that the airfoil can be easily removed from the rotor disk through the opening without having to disassemble the turbo machine.
It is another object of the present invention to allow for a turbomachinery that uses less material and therefore is lighter in weight that other turbo machines.